Catadioptric Telescopes | Good things come in small packages

You read about catoptric (reflecting) and dioptric (refracting) telescopes in the previous posts. Now, we will study the third kind – catadioptric. The term, as is evident, is a portmanteau of the former two terms (catoptric + dioptric = catadioptric), not just literally but also technically. A catadioptrictelescope is one where refraction and reflection are combined, via lenses (dioptrics) and mirrors (catoptrics) in the objective. Catadioptric telescopes, in a manner of speaking, bring both the worlds together into a compacter form factor.

Basic design

Catadioptric telescopes combine specifically shaped mirrors and lenses to form an image. This is usually done so that the telescope can have an overall greater degree of error correction than their lens or mirror counterparts with a consequently wider aberration-free field of view (also field of vision, abbreviated FOV, is the extent of the observable world that is seen at any given moment). Their designs can have simple all spherical surfaces and can take advantage of a folded optical path that reduces the mass of the telescope, making them easier to manufacture. Remember the Cassegrain design of reflectors from the post “Reflecting Telescopes“? These telescopes are conceptually similar and in fact extend from similar principles.

Many design variations of catadioptric telescopes employ correctors, a lens or curved mirror in a combined image-forming optical system so that the reflective or refractive element can correct the aberrations produced by its counterpart.

Problems with the catadioptric design

As you will see soon, these telescopes are not perfect either and suffer from problems of their own. However, the problems, in the most preferred versions of these telescopes, are not deal-breakers because they can be fixed by using the right enhancements or accessories (discussed later). Also, these problems are overshadowed by a very important advantage which is compactness. However, that statement should not be construed as an endorsement of these telescopes, in any way. Trust me, your decision could sway in any way when it comes to choosing the right telescope because each kind of telescope, be it refractor, reflector or catadioptric, is unique and carries it’s own set of advantages and drawbacks. What matters most to an astrophysicist, may mean something to you, and probably nothing to me. So, make an informed decision. We will study the comparisons, in the post The showdown, which would hopefully help you make the right choice.

Design variations

Just like reflectors and refractors, catadioptric telescopes too have been experimented with, in terms of design. Let’s see how.

Catadioptric dialytes

Catadioptricdialytes are the earliest type of catadioptric telescope. They consist of a single element refractor objective combined with a silver-backed negative lens. They are bygone now. More advanced catadioptric telescopes designs replaced the dialytes design.[1]

Full-aperture correctors

As mentioned, previously, there are several catadioptric telescope designs that take advantage of placing a full-aperture lens (commonly called a “corrector plate“, full diameter meaning same size as the primary) in front of a spherical primary mirror. These telescopes work by combining a spherical mirror’s ability to reflect light back to the same point with a large lens at the front of the system (a corrector) that slightly bends the incoming light, allowing the spherical mirror to image objects at infinity. This design has been adapted to create compact long focal length catadioptric Cassegrains, as we will see soon.[2]

The Schmidt corrector, the first full-aperture corrector plate made, was used in Bernhard Schmidt’sSchmidt telescope (popularly known as the Schmidt camera). A Schmidt corrector plate is an aspheric lens which is designed to correct the spherical aberration in the spherical primary mirror it is combined with. The corrector’s complex shape takes several processes to make, starting with a flat piece of optical glass, placing a vacuum on one side of it to curve the whole piece, then grinding and polishing the other side flat to achieve the exact shape required to correct the spherical aberration caused by the primary mirror. The design has lent itself to many Schmidt variants. Let’s see the most important ones.[3]

The Schmidt–Cassegrain is a full aperture catadioptric telescope that combines a Cassegrain reflector’s optical path (described in the post “Refracting Telescopes“) with a Schmidtcorrector plate to make a compact astronomical telescope. This telescope consists of a convex secondary mirror acting as a field flattener that relays the image through the hole in the primary mirror to a final focal plane located behind the primary.[4]

The Schmidt–Cassegrain design is very popular because it allows achieving long focal length of a refracting telescope with large aperture of a reflecting telescope, in a compacter form. However, owing to the long focal length of the objective, these telescopes have narrower field of view (or FOV) compared to reflectors (but higher magnification). Why? Because, if you remember what we discussed in the post “Rudiments of Telescopes” about magnification, longer the focal length of the objective (assuming fixed focal length of the eyepiece), higher is the magnification but that means smaller field of view (remember higher magnification is not always useful). However, as we will see in one of the subsequent posts on accessories, choosing the right eyepiece (with lower focal length) can help getting a wider field of view. I will refresh your memory of the formula for magnification here, if you feel too lazy to go to that post (I would strongly encourage you to read that post, if you haven’t already).

where fo is the focal length of the objective and fe is the focal length of the eyepiece.

Schmidt–Newtononian

A Schmidt–Newtonian orSchmidt–Newton telescope combines elements from both the Schmidt camera or telescope and the Newtonian reflector. In this telescope design, a spherical primary mirror is combined with a Schmidt corrector plate, which corrects the spherical aberration. The resulting system has less coma than a reflecting telescope with a parabolic mirror (which is free of spherical aberration but not free of coma). The design uses a 45° flat secondary mirror to view the image, as in a standard Newtonian reflector telescope.[5]

As you might have already noticed about the Schmidt correctors, the complexity in manufacturing these plates can be a pain. Another type of full aperture corrector designs, the Meniscus corrector shell, tries to overcome that.

The idea of replacing the complicated Schmidt corrector plate with an easy to manufacture full aperture spherical meniscus lens (a meniscus corrector shell) and to create a wide field telescope occurred to many optical designers in early 1940s, including Albert Bouwers, Dmitri Dmitrievich Maksutov among others.[6]

So, what is meniscus corrector? It is basically a negative meniscus lens. Remember the types of spherical lenses from the post “Optical Elements“? Never mind if you don’t. Here it is again.

Meniscus is a combination of convex and concave surfaces. It can be either positive or negative, depending on the relative curvatures of the two surfaces. A negative meniscus lens has a steeper concave surface and is thinner at the center than at the periphery (negative because it is diverging, owing to the concave surface being steeper of the two).

Albert Bouwers built a prototype meniscus telescope, known as the Bouwers meniscus telescope, that used a spherically concentric meniscus, based on the earlier catadioptric telescope, the Schmidt camera or Schmidt telescope. It had the spherical mirror and spherical meniscus corrector shell, all with a common radius of curvature (a concentric or monocentric design. Like the Schmidt camera, the meniscus telescope has the aperture stop coincide with the center of curvature. The design has an ultrawide field of view with no spherical aberration but does not correct chromatic aberration and was only suitable as a monochromatic astronomical camera working at a single wavelength of light. Bouwers, later, came up with a design that used a cemented doublet to form the meniscus corrector shell to correct chromatic aberration.[7]

Dmitri Maksutov built a prototype for a similar type of meniscus telescope, called the Maksutov telescope. His design corrected spherical and chromatic aberrations by placing a weak negative meniscus corrector closer to the primary mirror. Maksutov based his design on the idea behind the Schmidt camera of using the spherical errors of a negative lens to correct the opposite errors in a spherical primary mirror.[8]

Maksutov Newtonians

These telescopes use the Maksutov design in Newtonian configuration (Newtonian reflectors). They have minimal aberration over a wide field of view, with one-fourth the coma of a similar standard Newtonian and one-half the coma of a Schmidt-Newtonian. Diffraction can also be minimized by using a high focal ratio with a proportionally small diagonal mirror mounted on the corrector, allowing this design to present contrast and image quality approaching that of unobstructed high-end refractors. These are not widely popular and, hence, details can be left out.[9]

The most popular variation of the Maksutov telescope is the Maksutov–Cassegrain, that combines the principles of Maksutov and Cassegrain (from the Cassegrain reflectors). Remember, the Cassegrain aspect, fixes the off-axis aberrations like coma.

Matter-of-factly, there are many Maksutov designs that use a Cassegrain configuration, mounting a convex secondary mirror near the focus of the primary mirror. Most types use full-aperture correctors and are therefore not very large, since the corrector plate rapidly becomes prohibitively large, heavy and expensive as the aperture increases, with very long cool-down times to reach optimal optical performance. Most commercial manufacturers usually stop at 180 mm (7 in).[10]

Gregory or “spot” Maksutov–Cassegrains

Maksutov had explored the possibility of a ‘folded’ Cassegrain-type construction with a secondary silvered “spot” on the convex side of the meniscus facing the primary mirror. He thought this would create a sealed and rugged optical system. Most Maksutovs manufactured today are of this type (called either a Gregory–Maksutov or Spot-Maksutov“), using all-spherical surfaces and have a small aluminized spot on the inner face of the corrector, as a secondary.

The advantage is simpler design. The disadvantage is that, if all spherical surfaces are used, such systems have to have focal ratios above f/15 to avoid aberrations. Also, the secondary being a spot instead of an element, the freedom of fine-tuning the system by changing the radius of curvature of the secondary is lost. Gregory himself, in a second f/15 design, resorted to aspherization of the front corrector surface (or the primary mirror) in order to reduce aberrations. This has led to other designs with aspheric or additional elements to further reduce off-axis aberration. This type of Maksutov-Cassegrain’s high focal ratio and, thus, narrower field of view makes them more suitable for lunar and planetary imaging and any other type of observing where a narrow field high power view is a plus, such as resolving tightly packed clusters and double stars. Remember the formula of magnification? Higher focal ratio, with fixed fixed focal length of the eyepiece, means higher focal length of the objective and, hence, higher magnification and, thus, narrower field of view.[11]

Rutten Maksutov–Cassegrain

The Rutten Maksutov–Cassegrain (also called a Rumak or Sigler Maksutov) solves the problem of not being able to tune the secondary as in the Spot-Maksutov, owing to a separate secondary mirror mounted on back of the meniscus corrector, sometimes similar to the corrector/mirror holder configurations found in commercial Schmidt–Cassegrains. This provides an extra degree of freedom in correcting aberration by changing the curvature of the corrector and the secondary independently. Specifically it allows the designer to aspherize the secondary to provide a much wider flat field than the traditional spot Maksutovs, with less off-axis coma. This version is named after the work of Dutch optical designer Harrie Rutten.[12]

Sub-aperture corrector Maksutov–Cassegrains

Maksutov noted in his designs that, instead of a full-aperture corrector, using a small sub-aperture corrector placed in the converging light cone of the primary mirror can achieve the same effect. In the 1980s, Dave Shafer and Ralph W. Field came out with sub-aperture Maksutov-Cassegrain designs based on this idea. The design saves on the mass and cool-down (thermal-stabilization with respect to the outside temperature) time of a full aperture corrector. It has the drawbacks of an open, unsealed tube and requires a spider assembly to hold the secondary mirror and corrector, which inevitably affects image quality through diffraction artifacts. Also since the light passes through the corrector twice, the number of surfaces involved is multiplied, making it difficult to achieve good aberration correction.[13]

Sub-aperture correctors

Sub-aperture correctorcatadioptric telescopes encompass the more specific Sub-aperture corrector Maksutov-Cassegrains (discussed above). In principle, in these telescopes, the corrector plate is smaller than the objective (hence sub-aperture) and usually at the focus of the it. These elements can be both lenses and mirrors, but since multiple surfaces are involved, achieving good aberration correction in these systems can be very complex. Examples of sub-aperture corrector telescopes are the Argunov–Cassegrain, the Klevtsov–Cassegrain and the sub-aperture corrector Maksutov-Cassegrain (which use a secondary mirror, in addition) telescopes. If you are wondering why the Argunov–Cassegrain and Klevtsov–Cassegrain telescopes don’t have a secondary mirror (which is a characteristic of a classical Cassegrain), well, Argunov and Klevtsov decided to replace the secondary mirror by a special sub-aperture secondary corrector group.[14] We will see each in detail below.

The Argunov–Cassegrain telescope design was first introduced by P. P. Argunov. All optics are spherical, and the classical Cassegrain secondary mirror is replaced by a sub-aperture secondary corrector group consisting of three air-spaced elements, two lenses and a Mangin mirror (see below).[15]

Argunov systems only employ spherical surfaces and avoid the practical difficulties of making and testing aspheres (as in the Schmidt corrector). However, this seeming benefit is marginal, as it is almost as difficult to make a true, zone-free sphere of precise radius of curvature as it is to make an asphere of equivalent precision. Also, since multiple surfaces are involved, creating a design with good aberration correction can be very complex. These telescopes are rarely used for stargazing, so I will keep out the further details from this post.

A Manginmirror is a negative meniscus lens with the reflective surface on the rear side of the glass forming a curved mirror that reflects light without spherical aberration.

The Klevtsov–Cassegrain telescope, similarly, is a sub-aperture corrector telescope that uses a spherical primary mirror and a sub-aperture secondary corrector group composed of a small lens and a Mangin mirror. All of the optical surfaces are spherical or near-spherical. The secondary Mangin mirror and the meniscus corrector are held in place by a spider vane and the front of the telescope tube is otherwise open.[16] This design was originally envisaged by G. I. Popov (who apparently did not get credit) with a practical implementation by Yuri A. Klevtsov.

These types of telescopes have the disadvantage of spider to hold the corrector causing diffraction artifacts and, since multiple surfaces are involved just like in the Argunov cousin, achieving good aberration correction can be very complex. Again, due to it’s rare use in stargazing these days, we will keep it out of further discussion.

Out of the above designs, Schmidt-Cassegrain and Maksutov-Cassegrain are the most popular and widely used catadiotric telescopes, for stargazing, today.

In the next post, The Showdown, we will see how the major types of telescopes stack against each other.